So-called dc to ac converters, also known as inverters, are used to drive electric motors with high voltage and thus high internally stored intermediate circuit energy, especially in vehicles with onboard network voltages over 60 V. From this voltage range onward, one speaks in the automotive industry of so-called high voltage (HV) applications. One such application is, for example, an inverter for an electrically operated refrigerant compressor in a vehicle.
High voltage circuits, especially in electric or hybrid vehicles, also use so-called intermediate circuit capacitors in order to temporarily store electric energy in the high voltage circuit of the inverter.
These intermediate circuit capacitors are often designed to make use of electrolytic capacitors. Such electrolytic capacitors have the advantage of a high energy density, a large operating temperature range, and low costs.
The electrolytes used in electrolytic capacitors are at present available for maximum voltages up to 450 V. Hence, this type of capacitor can be used as a single capacitor in electric circuits on account of the safety margin of up to around 400 V which needs to be observed.
If higher voltages than 400 V are to be stored in an intermediate circuit, such as a voltage of 600 V, several capacitors are often arranged in series (a series circuit) for this.
For identical ideal components such as capacitors, the voltage in the series circuit would be evenly divided among the series connected capacitors. In the example of a series circuit with two capacitors C1 and C2 ideally the intermediate circuit voltage applied of 600 V would be divided so that 300 V apiece is applied across the capacitors C1 and C2.
But in practice this is seldom the case, owing to parasitic properties of a real capacitor. Thus, unless suitable countermeasures are taken, there is a danger that one of the capacitors C1 or C2 will be loaded more heavily than the other and thus may be ruined by an excess voltage.
The reason for this uneven voltage division between the capacitors C1 and C2 lies in a so-called leakage current Ileak.
For a uniform division of the voltage drops across the series connected capacitors, so-called “balancing or symmetrization”, symmetry resistors are used in the prior art. These symmetry resistors are each arranged in parallel with the associated capacitors and are dimensioned such that the current I through these resistors is large in relation to the leakage current Ileak of the capacitors. Thus, the voltage drop across the symmetry resistor as well as that across the parallel switched capacitor will be determined by the larger current I and can thus be set at an equal value.
Another possibility of voltage balancing, or a uniform division of the voltage drops across all components placed in the series circuit, is to use transistors in a so-called emitter follower circuit. Usually, for two capacitors one also uses two transistors in order to equalize the voltages.
The drawback to this solution is that in this case as well a balancing current flow through the semiconductor which is larger than the leakage current Ileak of the electrolytic capacitors used.
The drawback to these known solutions from the prior art is the constant current flow through the symmetry resistors or semiconductors. This occurs even when the voltage drop at the series connected capacitors is equal and no balancing is necessary. As a result, energy is consumed by the unused current flow and thus heat is generated and needs to be dissipated.
As a result, uneconomically, the components for the balancing need to be designed according to the heat losses, which makes them large in size and more costly.
The problem which the invention proposes to solve is to indicate a method for the voltage balancing of series connected capacitors with which a voltage in an intermediate circuit of an electric circuit can be easily and safely balanced among several series connected capacitors and thus these components are operated safely in terms of their voltage strength.